This application claims priority from Japanese Patent Application Nos. 2002-174938 filed Jun. 14, 2002 and 2003-020560 filed Jan. 29, 2003, which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a wavelength converter and a wavelength converting apparatus, and more particularly to a wavelength converter and a pump wavelength variable type wavelength converting apparatus that can be designed to handle a given number of pump wavelengths, can prevent reduction in a conversion efficiency, and can be simply configured using a practical size nonlinear optical material.
2. Description of the Related Art
Conventionally, wavelength converters and wavelength converting apparatuses configured using them have been known utilizing a variety of second order nonlinear optical effects. For example, a second harmonic generation apparatus can convert incident light to light (second harmonics) with half the original wavelength (twice the frequency). A sum frequency generation apparatus can convert two light beams with different wavelengths into a light beam with a frequency corresponding to the sum frequency of the two frequencies.
On the other hand, difference frequency generation apparatus can convert two light beams with different wavelengths into a light beam with a frequency corresponding to the difference frequency between the two frequencies. In addition, when one of the incident light beams is larger enough than the other of them, it can be configured as an optical amplifier that amplifies the intensity of the incident light utilizing a parametric effect. It is also applicable as a wavelength tunable light source by configuring a parametric resonator utilizing the parametric effect.
Next, the operation principle of conventional wavelength converters will be described briefly by way of example of difference frequency generators utilizing the second order nonlinear optical effect. These converters convert signal light with a wavelength xcex1 to idler light with a wavelength xcex2 by launching the signal light to a nonlinear optical medium pumped by pump light with a wavelength xcex3. The following equation is capable of coping with the three wavelengths, including the case where xcex1=xcex2.                               1                      λ            3                          =                              1                          λ              1                                +                      1                          λ              2                                                          (        1        )            
Research and development of various materials have been conducted as nonlinear optical media capable of coping with such elements. As for element structures, the so-called xe2x80x9cquasi-phase match type structurexe2x80x9d is considered to be promising as reported by M. H. Chou, et al., (Optics Letters, Vol. 23, p. 1004 (1998)), for example. It has a structure that causes a second order nonlinear optical material such as LiNbO3 to vary (modulate) its nonlinear optical coefficient periodically at a uniform period.
FIGS. 1A and 1B are diagrams for explaining a conventional wavelength converter (difference frequency generator) utilizing the second order nonlinear optical effect: FIG. 1A is a diagram illustrating a configuration of the wavelength converter conceptually; and FIG. 1B is a diagram illustrating the dependence of a conversion efficiency on a phase mismatch amount. To create a quasi-phase match type structure in a second order nonlinear optical material, the following methods are conceivable: First, a method of carrying out periodical modulation by spatially, alternately reversing the sign of the nonlinear optical coefficient of the material; second, a method of carrying out the periodical modulation by alternately placing sections with large and small nonlinear optical coefficients.
As for a ferroelectric crystal such as LiNbO3, the sign of the nonlinear optical coefficient (d constant) corresponds to the polarity of the spontaneous polarization. Thus, in the wavelength converter shown in FIG. 1A, an optical waveguide 12 is formed in a nonlinear optical medium, a LiNbO3 substrate 11, by a proton exchange method to periodically reversing the spontaneous polarization of the LiNbO3 at a modulation period (modulation period of the nonlinear optical coefficient) xcex90=14.75 xcexcm, thereby modulating the nonlinear optical coefficient. The wavelength converter is supplied with signal light 13 and pump light 15 via a multiplexer 17. The wavelength converter can carry out the wavelength conversion of the 1.55 xcexcm band signal light 13 by the 0.78 xcexcm band pump light 15.
In such a converter, the phase mismatch amount xcex94xcex2 is given by the following equation.                               Δ          ⁢                      xe2x80x83                    ⁢          β                =                  2          ⁢                      π            ·                          (                                                                    n                    3                                                        λ                    3                                                  -                                                      n                    2                                                        λ                    2                                                  -                                                      n                    1                                                        λ                    1                                                              )                                                          (        2        )            
where n1 is the refractive index of the LiNbO3 for the signal light 13 with the wavelength xcex1; n2 is the refractive index for idler light (difference frequency light) 14 with the wavelength xcex2; n3 is the refractive index for the pump light 15 with the wavelength xcex3; and xcex90 is the modulation period of the nonlinear optical coefficient. The conversion efficiency xcex7 is given by the following equation using the phase mismatch amount xcex94xcex2.                     η        =                              η            max                    ·                                    {                                                sin                  ⁡                                      [                                                                  (                                                                              Δ                            ⁢                                                          xe2x80x83                                                        ⁢                            β                                                    -                                                                                    2                              ⁢                              π                                                                                      Λ                              0                                                                                                      )                                            ·                                              L                        2                                                              ]                                                                    [                                                            (                                                                        Δ                          ⁢                                                      xe2x80x83                                                    ⁢                          β                                                -                                                                              2                            ⁢                            π                                                                                Λ                            0                                                                                              )                                        ·                                          L                      2                                                        ]                                            }                        2                                              (        3        )            
where L is the length of the nonlinear optical medium in the waveguide direction. Accordingly, the conversion efficiency xcex7 of the wavelength converter takes the maximum value when the phase mismatch amount xcex94xcex2 is 2xcfx80/xcex90. For example, consider the case where the wavelength xcex1 of the signal light 13 is fixed. In this case, the wavelength of the pump light 15 that satisfies the xe2x80x9cquasi-phase matching conditionxe2x80x9d, in which the phase mismatch amount xcex9462  given by the foregoing equation (2) becomes 2xcfx80/xcex90, depends on the chromatic dispersion of the refractive index of the nonlinear optical medium, and is determined uniquely if the modulation period xcex90 is given.
Varying the wavelength of the pump light 15 from the quasi-phase match wavelength that satisfies the quasi-phase matching condition, the conversion efficiency xcex7 reduces according to the foregoing equations (2) and (3). FIG. 1B is a graph illustrating the dependence of the conversion efficiency xcex7 on the phase mismatch amount xcex94xcex2 in which the conversion efficiency xcex7 is normalized in such a manner that the maximum value becomes one. Assume that the length of the optical waveguide 12 of the wavelength converter consisting of the LiNbO3 is 42 mm. Then, the band of the phase mismatch amount xcex94xcex2 in which the conversion efficiency xcex7 becomes half the maximum value is very narrow of about 0.1 nm in terms of 0.78 xcexcm band pump wavelength.
As is clear from the foregoing equation (1), a plurality of pump light beams with different wavelengths are required to convert the wavelength xcex1 of the signal light 13 to the difference frequency light with a given wavelength (xcex2xe2x80x2) However, the conventional modulation structure as illustrated in FIG. 1A, in which the nonlinear optical coefficient varies periodically at a uniform period, cannot vary the wavelength of the pump light substantially because of the narrow allowable range of the wavelength of the pump light. As a result, it cannot achieve the conversion to the difference frequency light with a given wavelength.
Next, to handle the different pump light wavelengths, a method is also possible in which modulation structures with a plurality of different modulation periods are disposed sequentially in the longitudinal direction. However, when the total length of the nonlinear optical media is fixed, the length of a nonlinear optical medium used in each modulation period is reduced. Generally, the conversion efficiency xcex7 of the wavelength converter utilizing the second order nonlinear optical effect is proportional to the square of the length of the nonlinear optical medium. Accordingly, disposing four types of the modulation periods will reduce the conversion efficiency xcex7 to 6.25% as compared with the case where the nonlinear optical medium with the same length is used.
To configure a wavelength converter capable of coping with a plurality of pump light wavelengths, a method of providing a periodically modulated structure with a phase reversal structure is proposed by M. H. Chou et al. (Optics Letters, Vol. 24, p. 1157 (1999)).
FIGS. 2A and 2B are diagram illustrating a conventional wavelength converter capable of coping with a plurality of pump light wavelengths by providing a phase reversal structure to a periodically modulated structure: FIG. 2A is a plan view schematically showing a configuration of the wavelength converter; and FIG. 2B is an enlarged view of its portion. In addition, FIGS. 3A-3F are graphs illustrating the behavior of the phase reversal in the wavelength converter, and the dependence of the conversion efficiency on the phase mismatch amount.
As the wavelength converter shown in FIG. 1A, the wavelength converter forms an optical waveguide 22 in a LiNbO3 substrate 21 used as the nonlinear optical medium by the proton exchange method, and provides the modulation to the nonlinear optical coefficient by periodically reversing the spontaneous polarization of the LiNbO3 at a fundamental modulation period xcex90=14.75 xcexcm. More specifically, the wavelength converter forms a phase reversal structure by reversing by an amount of 180 degrees the phase of the polarization reversal structure, which has a fixed fundamental period xcex90 (fundamental modulation period of 14.75 xcexcm), at a longer uniform period xcex9ph, thereby enabling the conversion efficiency xcex7 to have peaks at a plurality of phase mismatch amounts xcex94xcex2. Incidentally, using the pump light 25 with the wavelength xcex3 in the 0.78 xcexcm band incident via the multiplexer 27, the wavelength converter can also achieve the wavelength conversion of the signal light 23 with the wavelength xcex1 in the 1.55 xcexcm band incident via the same multiplexer 27 into the difference frequency light 24 with the wavelength xcex2.
FIG. 3A is a diagram illustrating the phase variation in the longitudinal direction in a nonlinear optical medium with a polarization reversal structure having the phase reversal with a phase reversal period xcex9ph of 14 mm and a duty factor of 50%. FIG. 3B is a diagram illustrating the dependence of the conversion efficiency on the phase mismatch amount, which is normalized with respect to the conversion efficiency of a wavelength converter which uses a nonlinear optical medium with the same length as the nonlinear optical medium shown in FIG. 1A, but without the phase reversal structure. In the wavelength converter, the length of the optical waveguide in which the polarization reversal is formed is 42 mm.
As seen from FIG. 3B, the conversion efficiency becomes maximum when the phase mismatch amount xcex94xcex2 is {(2xcfx80/xcex90)xe2x88x92(2xcfx80/xcex9ph)} and {(2xcfx80/xcex90)+(2xcfx80/xcex9ph)}, which indicates that the two pump wavelengths can be used for the wavelength conversion.
In addition, as illustrated in FIGS. 3C and 3D, setting the period xcex9ph at 7 mm and the duty factor of the phase reversal at 26.5% makes the conversion efficiency maximum when the phase mismatch amount xcex94xcex2 equals {(2xcfx80/xcex90)xe2x88x92(2xcfx80/xcex9ph)}, (2xcfx80/xcex90) and {(2xcfx80/xcex90)+(2xcfx80/xcex9ph)}, thereby enabling the wavelength conversion to use the three pump wavelengths.
Furthermore, as illustrated in FIGS. 3E and 3F, superimposing two phase reversal with period xcex9ph and 2xcex9ph make the conversion efficiency maximum when the phase mismatch amount xcex94xcex2 is {(2xcfx80/xcex90)xe2x88x92(6xcfx80/xcex9ph)}, {(2/xcex90)xe2x88x92(2xcfx80/xcex9ph)}, {(2xcfx80/xcex90)+(2xcfx80/xcex9ph)} and {(2xcfx80/xcex90)+(6xcfx80/xcex9ph)}. Thus, four peaks are obtained at every 4xcfx80/xcex9ph interval, which enables the wavelength conversion using the four pump wavelengths.
However, the conventional wavelength converter with the foregoing configuration has the following problems.
First, the normalized conversion efficiency of the structure with the four peaks as illustrated in FIG. 3F brings about spurious secondary peaks of the conversion efficiency other than the desired wavelengths. As a result the conversion efficiency of the converter is reduced down to 17%.
Second, trying to obtain the peaks of the conversion efficiency at narrow pump light wavelength intervals inevitably requires a long phase reversal structure. This offers a problem of imposing restrictions on placing the phase reversal period in a widely used substrate with the size of 3-4 inches.
The peak intervals of phase matching curves of FIGS. 3A, 3C and 3E are 0.8 nm in terms of the pump wavelength in the 0.78 xcexcm band, which means that the pump wavelength is variable at 400 GHz intervals. More specifically, from the relationship of equation (1), varying the pump light wavelength brings about the variation of the idler light wavelength by an amount of the variation of the pump light wavelength, which means that the idler light wavelength can be varied at the intervals of 400 GHz.
Considering the application to the WDM communications, devices with narrower intervals such as 200 GHz and 100 GHz will be required. For example, the normalized conversion efficiency of the structure with the four peaks as illustrate in FIG. 3F can narrow the peak intervals by increasing the phase reversal period because the phase mismatch amount xcex94xcex2 has the peaks at every 4xcfx80/xcex9ph interval for the phase reversal period xcex9ph. Considering the case where the LiNbO3 optical waveguide handles the pump wavelengths with the intervals of 200 GHz or 100 GHz, the phase reversal period required for the four pump wavelengths becomes very long such as 28 mm and 56 mm.
Third, although the method of handling the number of pump light wavelengths from one to four by superimposing the phase reversal pattern is disclosed in the foregoing document as described above, a method of handling the other number of the pump light wavelengths is unknown. Consequently, it is difficult to handle a desired number of pump light wavelengths flexibly.
As a result of studying device structures enabling highly efficient, multi-wavelength pumping, the inventors of the present invention discover a device structure capable of coping with a desired number of pump wavelengths without much losing the efficiency by introducing, into a periodically modulated structure of a nonlinear optical coefficient, a structure in which a continuous frequency modulation structure or phase modulation structure is repeated at a uniform period, and by optimizing a modulation curve of the frequency modulation structure or phase modulation structure. The device structure can provide a wavelength converter and pump wavelength variable type wavelength converting apparatus that enables a design capable of coping with a desired number of the pump light wavelengths, and that can prevent the reduction in the conversion efficiency and can be configured easily using a practical size of a nonlinear optical material.
According to a first aspect of the present invention, there is provided a wavelength converter comprising: a nonlinear optical medium having a frequency modulated periodically modulated structure composed of a modulation unit structure and a frequency modulation structure, the modulation unit structure having a structure in which a nonlinear optical coefficient is modulated periodically at a period nearly equal to a fundamental period xcex90 varies nearly continuously, and the frequency modulation structure having a structure in which the modulation unit structure is repeated at a frequency modulation period xcex9f longer than the fundamental period xcex90; and means for launching light with one or two wavelengths of three wavelengths xcex1, xcex2 and xcex3 including xcex1=xcex2, which satisfy the following equation, onto the nonlinear optical medium,                               1                      λ            3                          =                              1                          λ              1                                +                      1                          λ              2                                                          (        4        )            
wherein the wavelength converter, utilizing a second order nonlinear optical effect occurring in the nonlinear optical medium, converts the input light into outgoing light with a wavelength equal to one of the three wavelengths, and different from at least one of the wavelengths of the incident light.
According to a second aspect of the present invention, there is provided a wavelength converter comprising: a nonlinear optical medium having a phase modulated periodically modulated structure composed of a modulation unit structure and a phase modulation structure, the modulation unit structure having a structure in which a nonlinear optical coefficient is modulated periodically at a period substantially equal to a fundamental period xcex90 modulations varies nearly continuously, and the phase modulation structure having a structure in which the phase variation of the modulation unit structure is repeated at a phase modulation period xcex9ph longer than the fundamental period xcex90; and means for launching light with one or two wavelengths of three wavelengths xcex1, xcex2 and xcex3 including xcex1=xcex2, which satisfy the foregoing equation (4), onto the nonlinear optical medium, wherein the wavelength converter, utilizing a second order nonlinear optical effect occurring in the nonlinear optical medium, converts the input light into outgoing light with a wavelength equal to one of the three wavelengths, and different from at least one of the wavelengths of the incident light.
According to another aspect of the present invention, there is provided a wavelength converting apparatus comprising a pumping source capable of varying its oscillation wavelength or switching a plurality of oscillation wavelengths, and a wavelength converter in accordance with the present invention, wherein the wavelength converter is configured such that it generates difference frequency light in the nonlinear optical medium from input signal light supplied from outside, and incident light supplied from the pumping source; and converts a wavelength of the difference frequency light by selecting the wavelength of the pump light with a phase mismatch amount that will maximize a generation efficiency of the difference frequency light.
According to the first aspect of the present invention, a phase mismatch amount xcex94xcex2 may be determined such that it will maximize the conversion efficiency at 2xcfx80/xcex90xc2x12xcfx80i/xcex9f (i=0, 1, . . . , n, where n is a positive integer), at 2xcfx80/xcex90xc2x12xcfx80 (2i+1)/xcex9f (i=0, 1, . . . , n, where n is a positive integer), or at 2xcfx80/xcex90+2xcfx80i/xcex9f (i=m, m+1, . . . , n, where m and n are positive or negative integers satisfying |m|xe2x89xa0|n|). This makes it possible to provide a wavelength converter and wavelength converting apparatus that enable the design capable of coping with a desired number of pump wavelengths, and that can prevent the reduction in the conversion efficiency, and can be configured easily using a practical size of the nonlinear optical material.
According to the second aspect of the present invention, the phase mismatch amount xcex94xcex2 maybe determined such that it will maximize the conversion efficiency at 2xcfx80/xcex90xc2x12xcfx80i/xcex9ph (i=0, 1, . . . , n, where n is a positive integer), at 2xcfx80/xcex90xc2x12xcfx80 (2i+1)/xcex9ph (i=0, 1, . . . , n, where n is a positive integer), or at 2xcfx80/xcex90xc2x12xcfx80i/xcex9ph (i=m, m+1, . . . , n, where m and n are positive or negative integers satisfying |m|xe2x89xa0|n|. This makes it possible to provide a wavelength converter and wavelength converting apparatus that enable the design capable of coping with a desired number of pump wavelengths, and that can prevent the reduction in the conversion efficiency, and can be configured easily using a practical size of the nonlinear optical material.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.